Magnetic flake core



United States Patent 2,873,225 MAGNETIC FLAKE CORE Edmond'Adams, Silver Spring, John F. Haben, Hyattsville, and Albert M. Syeles, Silver Spring, Md., assignors to the United States of America as represented by the Secretary of the Navy N0 Drawing. Application May 20, 1957 Serial No. 660,429

4 Claims. ('Cl. 148-105) I inductance coils, electronic filter networks, resonant circuits, transformers and the like.

More specifically, the invention relates to a new magnetic core material and a method of producing said core material and cores from said material.

Prior art magnetic cores have usually been manufactured from finely divided molybdenum-Permalioy compacted with an insulation-binder into the desired core configuration. The molybdenum-Permalloy employed usually contains about 2% molybdenum, 81% nickel and the remainder iron. These 2-81 Mo-Permalloy powder cores have high permeability but also have high power losses. Mo-Permailoy cores have the added disadvantage in that they contain 81% of nickel which is relatively scarce, expensive and strategically impon tant. The manufacture of the molybdenum-Permalloy satisfactory alternate or substitute materials for use in magnetic cores which would not have these disadvantages of expense and procurement.

Compacted powder cores have been prepared utilizing tent iron base alloy. not, however, satisfactory substitutes for 281 Mo-Permalloy powder cores. The Sendust powder cores, though they have power losses substantially equal to the Permalloy cores, have permeability values considerably lower. Alfenol powder cores, while exhibiting permeability values comparable to those of the Permalloy powder cores, had considerably higher power losses.

The Sendust and Alfenol alloys, because of their high initial and maximum perm-eabilities, were considered ideal for use in powdered cores and because of their inherent brittleness, pulverization of them proved to be not too difiicult. However, a high permeability .value for a pressed powder core is dependent on a high compact density which in turn is dependent on the ability of the particles to be easily deformed. Sendust and Alfenol powders are extremely hard and resistant to deformation and consequently Sendust and Alfenol powder cores have low densities and are characterized by permeability values considerably lower than those for corresponding Permalloy cores.

For many years it has been known that considerable improvement in the permeability values of powdered cores could be realized if the powder particles could be converted into the form of flakes i. e. shaped particles whose length and width are greater than their thickness. Flake cores of iron have been manufactured and it was found that if the flaked particles could be layered uniformly in a die cavity and compressed in. such a manner as to retain a high interlaminary resistance, then the permeability would be improved and the eddy current losses lowered. The permeability would be improved because of the more favorable penetration of flux at various levels of the applied field. The eddy current losses would be lowered because of the greater number of air gaps present perpendicular to the plane of the pressed flake laminates.

Flaking of iron powder particles was found to be relatively simple because of the ductility of the iron. However, alloys of iron such as Alfenol and Sendust, which have magnetic been considered impossible to flake because of their inherent qualities of hardness, brittleness and lack of ductility. Iron flake cores, because of their high electrical losses, have not proved satisfactory for most pur poses when compared with other powder cores.

It is, therefore, an object of the present invention to provide a new and improved non-strategic magnetic material for use in the manufacture of magnetic cores and the like.

Another object is to provide a new method of manufacturing an improved magnetic material .for use in the manufacture of magnetic cores and the like.

A further object is to provide an improved method of manufacturing magnetic cores and the like.

A still further object is to provide a new and improved magnetic core.

Other objects and the attendant advantages of the invention will become apparent to those skilled in the art as the invention is disclosed in the following detailed description.

The above mentioned objects are achieved in accord ance with the invention of preparing flakes of Sendust powder and compacting these flakes into the desired core shapes. The improved magnetic properties of the flake Sendust cores over the present Sendust powder cores makes them the first non-strategic substitute for applications which now require powdered high nickel alloys such as 2-81 molybdenum-Permalloy.

The present invention provides a method of fabricating Sendust flake cores which have heretofore been unattainable. in spite of the extreme brittleness of the alloy, Sendust flake compacts have been prepared with permeability values almost three times those of Sendust powder cores and exceeding 281 Mo-Permalloy powder cores by 8 percent. Consistently equal or lower total loss factors were measured on all the Sendust flake cores.

In accordance with the invention Sendust powder is heated to about 500 C. and then rolled between preheated rollers at a temperature of about 200 C. A thin flake material results which because of its flake structure is called Flakenol by which name it will be hereafter referred to in this application.

Sendust alloy containing l -7% silicon, balance iron may be prepared by melting electrolytic iron under a vacuum of about 230 microns and then adding silicon and 50% ferro-aluminum to the aluminum, 7-14% molten iron. The melt is then poured into molds in an atmosphere of dry hydrogen or helium to form ingots.

The initial permeability of Sendust reach-es a sharp peak value at 9.6% Si and 5.4% A1. The peak value for maximum permeability is found in alloys of 9.7% Si and 6.2% Al. Any gross deviations from these compositions lead to reduced permeability values. However,

, rather than on the intrinsic permeability of in powdered and flake cores the effective permeability is more dependent on the number of air gaps present the alloy.

properties superior to iron, have Alloys composed of 47% Al, 7 to 14% Si and the balance iron have given satisfactory magnetic cores.

The Sendust alloy may be crushed and ground by conventional techniques to yield powder of the proper particle size. The Sendust ingot after crushing to approximately two inch diameter may be homogenized for two hours in air at 1000 C. Following this treatment, the material may be crushed in a jaw mill and further reduced in a disc crusher so that all the powder passes a 30 mesh screen. The coarse powder may then be pulverized in a steel ball mill using hardened steel balls so that all the powder would pass a 120 mesh screen. Particles of any desired size may be utilized since all the particles will be broken up and the flake size distribution will always be -30 to -325 mesh size. However, the preferred size is minus 120 plus 200 mesh. Also, it is to be understood that any conventional method, other than those disclosed, may be used to reduce the ingots into powder particles.

The flaking of the Sendust powder may be carried out by rolling the hot powder between two preheated rolls. The preferred temperature of the powder at the time of rolling is 200 C. If the powder is heated to approximately 500 C. and the rolls preheated to approximately 140 C., the preferred 200 C. temperature of the powder at the time of rolling may be main tained. Although a temperature of 200 C. is preferred, the rolling may be carried out at any tempera ture above 200 C. up to the melting point of the alloy.

The flaking may be done by rolling the powder in one or more passes in a 6 inch diameter standard 2- high laboratory rolling mill. The rolls are preheated to 140 C. and the powder heated to 500 C. in an inclined chute and vibrated into the warm rolls for to duction into flake.

The flake material resulting from the rolling process may then be sieved to obtain a preferred size distribution or used as is. The flakes resulting from the rolling of Sendust powder of the preferred size of minus 120 plus, 200 mesh rolled out more than two passes on the rolling mill described above resulted in flakes of a size of minus 30 plus 325 mesh. The chief factor in determniing high permeability and low loss values in a compressed flake core is the thickness of the flake. The average flake thickness should not exceed 25 microns otherwise the permeability and losses will not be at the optimum value. Also a breath to thickness ratio greater than 4:1 is desirable for high permeability values.

The Sendust powder or Flakenol is given a low temperature anneal to relieve the strains of working and to pro-insulate by oxidizing the surface of the flakes. The flakes are best heated to a temperautre below the melting point of the Flakenol in a hydrogen atmosphere at a pressure of one atmosphere with the hydrogen flow being at a rate of 8 cubic feet per hour although the rate of flow of the hydrogen is not critical. During the heat treatment in the hydrogen atmosphere, the residual oxygen impurity in the hydrogen is suflicient to form a thin oxide coating primarily aluminum oxide on the surface of the flakes. The heat treatment could be carried out in air but the formation of the oxide coating under this condition is difficult to control.

The temperature used and the time required for the heat treatment in the hydrogen atmosphere depends on the properties desired in the final core. The temperature range which can be utilized is 400-900 C. with periods of 15 minutes to four hours. atures require less time and lower temperatures require more time within the range specified. For example, whcre a core having a permeability of 167 and a total loss at kc. of 400x10- is desired, the heat treatment in the hydrogen atmosphere is carried out at- 600 for l'houri Higher temper-- At the end of the heat treatment cycle, the flakes are cooled to room temperature still in the hydrogen atmosphere. At this point insulator binding material may be added to increase the strength and decrease powder losses. The addition of hydrolyzed tetra-ethy1- silicate in ethyl alcohol suflicient to leave a coating of silicon dioxide of one percent by weight of the flake mass has proven to be satisfactory yielding low loss cores of sufficient strength for handling and winding. Other insulating and binding materials may be employed such as sodium silicate, tctrabutyl' titana'te, magnesium methylate and aluminum isoproproxide. These materials may be utilized in any desired amounts from 0.15.0% consistent with high permeability and low power losses. For example, a small percentage of insulator-binder [0.1%] will give high permeability and somewhat higher losses while a larger percent [5.0%] will yield lower permeability and lower losses.

To obtain high permeability powder cores it is necessary to press the annealed and insulated Sendust flakes or Flakcnol at pressures of l00l25 tons per square inch. Besides, this requirement the flakes should be arranged systematically in the die cavity. To get a uniform layering, the flakes may be allowed to fall freely from a height of several inches while rotating the feed at a constant rate. The molding pressure is then applied at right angles to the plane of the flakes. Hardened three-section dies were required to withstand the high pressures and for ease of ejection of the pressed core. The fill factor has been found to be approximately 3:1 the same as for other related high permeability powders.

After the insulated flakes have been compacted to yield the shape suitable for the desired application, the shaped core is heated in a hydrogen atmosphere at atmospheric pressure. The temperature and time of the heat treatment should be such as to develop the desired: properties in the core. A temperature of 650 C. for onehalf hour is preferred. Higher temperatures yield erratic loss and permeability values. The core is then preferably cooled slowly to room temperature while still in the hydrogen atmosphere but may be removed from the annealing furnace while still hot, and air quenched if it is so desired.

The basic magnetic properties of cores formed in accordance with the invention may be measured by means of a single layer winding upon a toroidal specimen of the material. The permeability and total losses [eddy current, hysteresis and residual] can be determined from R. F. bridge measurements of the current, inductance and resistance of the toroidal winding. The permeability a of the core is:

where L is the inductance of the coil in henrys, N is the number of turns in the test winding, A is the cross section of the core in square centimeters and Dmis the mean diameter of the toroid in centimeters. The flux density B [ingaussl in the core is given by:

force, f is the frequency of alternatingcurrent in cycles per second, and R equals the efiective resistance of the core in ohms. Since Table I Loss Coeflieients [R/nLflX10 B=20 Material n X10 6X10 c l0 2 kn. 251m. 75 kc.

Mo-Permalloy 125 1. 0 19. 0 30 l00 430 1, 080 Do 26 6. 9 7. 7 96 370 700 Do 14 11. 4 7. 1 143 700 l, 030 Sendust Powder. 73 4. 6 8. 9 47 100 330 775 Flakenol 230 8. 2 3. 95 230 350 520 D0 204 10. 2 2: 1 20 165 275 380 Thus in spite of the extreme brittleness of the base alloy, utilizing the process of the invention Flakenol cores have been prepared with permeability values of 230, almost three times those of Sendust powder cores [80] and exceeding present 125p. 2-81 Mo-Permalloy powder cores by 80 percent.

From the foregoing it may be seen that there has been provided a novel magnetic material which can be considered as a non-strategic equivalent substitute for applications now using unstabilized 2-81 Mo-Permalloy powder cores. This substitution would release a considerable tonnage of nickel for more urgentuses. Since the permeability of the Flakenol cores ranges 50-80% higher than for present core materials and with equivalent low losses, miniaturization can be efiected resulting in a further saving of nickel. Because Flakenol cores have low eddy current loss coefficient and substantially higher permeability values, their use can be extended for applications at frequencies up to 100 kc., whereas present high permeability powder core materials are usually limited to use in frequencies up to 15 kc. The process of preparing Flakenol cores is simple and easy compared with 2-81 Mo-Permalloy cores and the same facilities used to compact Permalloy powder cores can be used for Flakenol cores. A simple one step insulation addition is possible consistent with high permeability and low losses.

It is understood that the invention may be practiced otherwise than as specifically described within the scope of the following claims.

We claim:

1. The method of preparing magnetic flake material for use in the manufacture of magnetic cores and the like comprising heating to about 500 C. a powder of an iron base alloy containing 4-7% aluminum and 7-14% silicon, the balance essentially iron, and rolling said hot powder between rolls preheated to C.

2. The method of preparing magnetic material for use '11 the manufacture of magnetic cores and the like comprising; rolling an iron base alloy containing 4-7% aluminum and -l4% silicon, the balance essentially iron, while maintaining the powder at a temperature of about 200 C. thereby to form flakes of the alloy, and heating the flakes at a temperature between 400 C. to 900 C. for a period of from 15 minutes to 4 hours in a hydrogen atmosphere to form a thin coating of oxide on said flakes.

3. A method of preparing magnetic material by rolling a powder of an alloy containing 4-7% aluminum, 7-l4% silicon, the balance essentially iron at a temperature from about 200 C. to the melting point of the alloy powder to form flakes of the alloy.

4. A method of preparing magnetic material for use in the manufacture of magnetic cores and the like comprising rolling at about 200 C. a powder of an alloy containing 4-7% aluminum, 7-14% silicon, the balance essentially iron, to form flakes of the powder.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Thurlby: Metal Progress, vol. 60, No. 4, October 1951, pages 83-87. Published by the American Society for Metals, Cleveland, Ohio.

The Making, Shaping and Treating of Steel, 6th edition, 1951, pp. 580-588 and 654. Published by the United States Steel Co., Pittsburgh, Pa.

Case et al.: Aluminum in Iron and Steel, 1953, pages 297-300. Published by John Wiley & Sons, Inc., New York, N. Y. 

3. A METHOD OF PREPARING MAGNETIC BY ROLLING A POWDER OF AN ALLOY CONTAINING 4-7% ALUMINUM, 7-14% SILICON, THE BALANCE ESSENTIALLY IRON AT A TEMPERATURE FROM ABOUT 200$ C. TO THE MELTING POINT OF THE ALLOY POWDER TO FORM FLAKES OF THE ALLOY. 